A measure of how much a substance reduces the speed of light waves passing through it. The index of refraction (or refractive index) of a substance equals the ratio of the speed of light in a vacuum to its speed in that substance. Lenses or materials with a high index of refraction slow down and refract (bend) light more than materials with a lower refractive index.

A measure of how mucha substance reduces the speed of light waves passing through it. The index of refraction (or refractive index) of a substance equals the ratio of the speed of light in a vacuum to its speed in that substance. Lenses or materials with a high index of refraction slow down and refract (bend) light more than materials with a lower refractive index.